1) Field of the Invention
This invention relates to an optical modulator suitable for use in the field of long distance optical communication systems.
2) Description of the Related Art
As data transmission rates have increased in recent years, optical modulators for modulating a data signal from an electric signal into an optical signal are being developed energetically in the field of long distance communication systems such as submarine optical communication.
An example of an optical modulators as just described is dual drive optical modulator 30 as shown in FIG. 22. Referring to FIG. 22, the dual drive optical modulator 30 shown includes a substrate 31 on which a Mach-Zehnder optical waveguide 32 is formed, and an electrode 33 formed integrally on the substrate 31 and including two signal electrodes 33A-1 and 33A-2 and a grounding electrode 33B. The dual drive optical modulator 30 modulates incoming light from a light source not shown with an NRZ data signal.
FIG. 23 is a sectional view taken along line A-Axe2x80x2 of the dual drive optical modulator 30 shown in FIG. 22. As seen in FIG. 23, the dual drive optical modulator 30 is configured such that the electrode 33 is integrally formed on the substrate 31, which is made of, for example, lithium niobate (LiNbO3) and cut in the Z-axis direction of the crystal orientation (Z-axis cut), together with the Mach-Zehnder optical waveguide 32.
The Mach-Zehnder optical waveguide 32 is formed by thermal diffusion of titanium (Ti) or a like substance on the substrate 31 and includes a Y branching waveguide 32A and two straight arm waveguides 32B-1 and 32B-2 on the light incoming side and a Y branching waveguide 32C on the light outgoing side.
The electrode 33 is formed partially on the substrate 31 with a buffer layer 35 (refer to FIG. 23) interposed therebetween and includes the two signal electrodes 33A-1 and 33A-2 and the grounding electrode 33B.
The electrode 33 can modulate incoming light into an NRZ optical signal by applying NRZ data signals from NRZ data signal generators 34A and 34B which are hereinafter described as electric signals to the signal electrodes 33A-1 and 33A-2.
As shown in FIG. 22, the signal electrodes 33A-1 and 33A-2 are each formed so as to establish an electric connection between two connector contacts on a one-side edge portion of the substrate 31 in its widthwise direction. Further, the signal electrode 33A-1 is formed such that part of it extends along and above the portion at which the straight arm waveguide 32B-1 is formed. Further, the grounding electrode 33B is formed such that it is disposed on the opposite sides of the signal electrodes 33A-1 and 33A-2 in a spaced relationship by a predetermined distance thereby to form a coplanar line on the substrate 31.
The NRZ data signal generator 34A applies a voltage signal (microwave) as an NRZ data signal to the signal electrodes 33A (33A-1 and 33A-2). The NRZ data signal generator 34B applies a voltage signal (microwave) as an NRZ data signal to the signal electrode 33B.
When light from a light source (not shown) is introduced into the dual drive optical modulator 30 having the configuration described above with reference to FIG. 22, while the light propagates in the Mach-Zehnder optical waveguide 32, it is modulated into an NRZ optical signal by the signal electrodes 33A-1 and 33A-2 to which a voltage signal (microwave) of NRZ data or the like is applied.
In order to design an optical device for which high speed operation is required such as an optical modulator described above, it is necessary as a basic design item to take several parameters into consideration including (1) the drive voltage, (2) the velocity match between the optical signal and the electric signal, (3) the attenuation constant of the electric signal, (4) the characteristic impedance (normally 50 xcexa9), (5) the wavelength chirp amount, and (6) the loss. Particularly, it is important for improvement in power consumption and transmission quality of the apparatus to lower the drive voltage of the optical device.
Where such a dual drive optical modulator as described above is used to modulate a voltage signal into a data optical signal of a transmission rate particularly of 10 Gb/s or more, preferably of approximately 40 Gb/s, it is a significant subject for improvement of the transmission quality to lower the drive voltage while arbitrating with the values of the other evaluation parameters such as the velocity match between the optical signal and the electric signal as described above.
It is an object of the present invention to provide an optical device which can lower the drive voltage which is used as one of the parameters for performance evaluation of the optical device.
In order to attain the objective described above, according to an aspect of the present invention, there is provided an optical device, comprising a substrate having an electro-optical effect and having formed thereon first and second ridges which extend in parallel to each other, first and second grooves which are positioned on the outer sides of the first and second ridges, respectively, a third groove which is positioned between the first and second ridges, and first and second banks which are positioned on the outer sides of the first and second grooves, respectively, a Mach-Zehnder optical waveguide formed on the substrate such that the Mach-Zehnder optical waveguide branches at a first Y branching waveguide into a first arm waveguide included in the first ridge and a second arm waveguide included in the second ridge and then joins together at a second Y branching waveguide, and electrode means formed on the substrate and including a first signal electrode formed on the first ridge, a second signal electrode formed on the second ridge and a grounding electrode formed on the first and second banks and the third groove for controlling light which propagates in the optical waveguide, the grounding electrode extending to the first groove adjacent to the first bank and the second groove adjacent to the second bank.
In the optical device, while incoming light propagates in the optical waveguide, an electric signal is applied to the electrode means to control the light which propagates in the optical waveguide. Since the grounding electrode which is a component of the electrode means extends to the first groove adjacent to the first bank and the second groove adjacent to the second bank, the drive voltage of the electric signal to be applied to the electrode means is lowered.
Accordingly, with the optical device, since the grounding electrode extends to the first groove adjacent to the first bank and the second groove adjacent to the second bank, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance. Consequently, the optical device is advantageous in that the transmission quality can be improved, power can be saved, and the performance of the optical modulator can be improved.
Preferably, in the optical device, a third bank is formed at a middle position between the first and second ridges in the third groove. In this instance, the grounding electrode may extend to the third groove adjacent to the opposite sides of the third bank, and the ridges may have a top face level substantially equal to that of the banks.
In the optical device, preferably the substrate is made of LiNbO3. The substrate made of LiNbO3 may be Z-axis cut. The ridges may have a top face level substantially equal to that of the banks. The grooves may have depths set substantially equal to each other.
In the optical device, preferably the signal electrodes contact with the respective corresponding ridges with a contact width smaller than the width of the ridges. Preferably, a buffer layer is formed between the substrate and the electrode means. The buffer layer may be formed in such a manner as to trace the shapes of the first, second and third grooves, or may be provided also in the first, second and third grooves and have, in the grooves, a thickness smaller than the thickness of a portion of the buffer layer which is present at any other location.
In the optical device, preferably an NRZ data signal generator for supplying an NRZ data signal is connected to the electrode means. A pad for external apparatus connection may be provided at each end portion of the electrode means and have a width equal to or smaller than 70 microns.
Another electrode may be provided at the second Y branching waveguide. In this instance, preferably a DC power supply is connected to the other electrode. A pad for external apparatus connection may be provided at each end portion of the other electrode and have a width equal to or smaller than 70 microns.
According to a second aspect of the present invention, there is provided an optical device, comprising a Z-axis cut substrate made of LiNbO3 and having formed thereon first and second ridges which extend in parallel to each other, first and second grooves which are positioned on the outer sides of the first and second ridges, respectively, a third groove which is positioned between the first and second ridges, and first and second banks which are positioned on the outer sides of the first and second grooves, respectively, a Mach-Zehnder optical waveguide formed on the substrate such that the Mach-Zehnder optical waveguide branches at a first Y branching waveguide into a first arm waveguide included in the first ridge and a second arm waveguide included in the second ridge and then joins together at a second Y branching waveguide, and electrode means formed on the substrate and including a first signal electrode formed on the first ridge, a second signal electrode formed on the second ridge and a grounding electrode formed on the first and second banks and the third groove for controlling light which propagates in the optical waveguide, a buffer layer being formed between the substrate and the electrode means, the grounding electrode extending to the first groove adjacent to the first bank and the second groove adjacent to the second bank.
Accordingly, with the optical device, since the grounding electrode extends to the first groove adjacent to the first bank and the second groove adjacent to the second bank, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance similar to the optical device according to the first aspect of the present invention. Consequently, the optical device is advantageous in that the transmission quality can be improved, power can be saved, and the performance of the optical modulator can be improved.
Further, the absorption loss of light propagating in the optical waveguide can be suppressed and the transmission quality can be improved by the buffer layer. Furthermore, since the substrate is made of LiNbO3 and Z-axis cut, the optical device is advantageous also in that four of the performance evaluation parameters upon configuration of a high speed optical modulator, that is, the drive voltage, the velocity match between the optical signal and the electric signal, the attenuation constant of the electric signal, and the characteristic impedance (50 xcexa9), can be matched readily.
According to a third aspect of the present invention, there is provided an optical device, comprising a substrate having an electro-optical effect and having formed thereon first and second ridges which extend in parallel to each other, first and second grooves which are positioned on the outer sides of the first and second ridges, respectively, a third groove which is positioned between the first and second ridges, first and second banks which are positioned on the outer sides of the first and second grooves, respectively, and a third bank which is positioned intermediately between the first and second ridges in the third groove in such a manner as to divide the third groove, a Mach-Zehnder optical waveguide formed on the substrate such that the Mach-Zehnder optical waveguide branches at a first Y branching waveguide into a first arm waveguide included in the first ridge and a second arm waveguide included in the second ridge and then joins together at a second Y branching waveguide, and electrode means formed on the substrate and including a first signal electrode formed on the first ridge, a second signal electrode formed on the second ridge and a grounding electrode formed on the first, second and third banks for controlling light which propagates in the optical waveguide, the grounding electrode extending to the third groove adjacent to the opposite sides of the third bank.
Accordingly, with the optical devices since the grounding electrode extends to the third groove adjacent to the opposite sides of the third bank, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance similarly to the optical device according to the first aspect of the present invention. Consequently, the optical device is advantageous in that the transmission quality can be improved and the power can be saved and besides the performance of the optical modulator can be improved.
Preferably, the substrate is made of LiNbO3, and the substrate made of LiNbO3 is Z-axis cut.
In the optical device, preferably the ridges have a top face level substantially equal to that of the banks, and the grooves have depths set substantially equal to each other.
In the optical device, preferably the signal electrodes contact with the respective corresponding ridges with a contact width smaller than the width of the ridges, and a buffer layer is formed between the substrate and the electrode means. The buffer layer may be formed in such a manner as to trace the shapes of the first, second and third grooves, or may be provided also in the first, second and third grooves and have, in the grooves, a thickness smaller than the thickness of a portion of the buffer layer which is present at any other location.
An NRZ data signal generator for supplying an NRZ data signal may be connected to the electrode means. Preferably, a pad for external apparatus connection is provided at each end portion of the electrode means and has a width equal to or smaller than 70 microns.
Another electrode may be provided at the second Y branching waveguide. Preferably, a DC power supply is connected to the other electrode. Further, preferably a pad for external apparatus connection is provided at each end portion of the other electrode and has a width equal to or smaller than 70 microns.
According to a fourth aspect of the present invention, there is provided an optical device, comprising a Z-axis cut substrate made of LiNbO3 and having formed thereon first and second ridges which extend in parallel to each other, first and second grooves which are positioned on the outer sides of the first and second ridges, respectively, a third groove which is positioned between the first and second ridges, first and second banks which are positioned on the outer sides of the first and second grooves, respectively, and a third bank which is positioned intermediately between the first and second ridges in the third groove in such a manner as to divide the third groove, a Mach-Zehnder optical waveguide formed on the substrate such that the Mach-Zehnder optical waveguide branches at a first Y branching waveguide into a first arm waveguide included in the first ridge and a second arm waveguide included in the second ridge and then joins together at a second Y branching waveguide, and electrode means formed on the substrate and including a first signal electrode formed on the first ridge, a second signal electrode formed on the second ridge and a grounding electrode formed on the first, second and third banks for controlling light which propagates in the optical waveguide, a buffer layer being formed between the substrate and the electrode means, the grounding electrode extending to the first groove adjacent to the first bank, the second groove adjacent to the second bank and the third groove adjacent to the opposite sides of the third bank.
Accordingly, with the optical device, since the grounding electrode extends to the third groove adjacent to the opposite sides of the third bank, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance similar to the optical device according to the first aspect of the present invention. Consequently, the optical device is advantageous in that the transmission quality can be improved, power can be saved, and the performance of the optical modulator can be improved.
Further, the absorption loss of light propagating in the optical waveguide can be suppressed and the transmission quality can be improved by the buffer layer. Furthermore, since the substrate is made of LiNbO3 and Z-axis cut, the optical device is advantageous also in that four of the performance evaluation parameters upon configuration of a high speed optical modulator, that is, the drive voltage, the velocity match between the optical signal and the electric signal, the attenuation constant of the electric signal, and the characteristic impedance (50 xcexa9), can be matched readily.
According to a fifth aspect of the present invention, there is provided an optical device, comprising a Mach-Zehnder first optical modulator including a first substrate having an electro-optical effect, a Mach-Zehnder first optical waveguide formed on the first substrate, and first electrode means formed on the first substrate for controlling light which propagates in the first optical waveguide, a Mach-Zehnder second optical modulator connected in cascade connection to the Mach-Zehnder first optical modulator and including a second substrate having an electro-optical effect, a Mach-Zehnder second optical waveguide formed on the second substrate and connected to the first optical waveguide, and second electrode means formed on the second substrate for controlling light which propagates in the second optical waveguide, a clock generator connected to a first one of the first and second electrode means for applying a clock signal to the first one of the first and second electrode means to produce an RZ signal, and an NRZ data signal generator connected to a second one of the first and second electrode means for supplying an NRZ data signal to the second one of the first and second electrode means, at least one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator being formed as an optical device having the characteristics of the optical device according to the first aspect of the present invention.
With the optical device, since at least one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed so as to have the characteristics of the optical device according to the first aspect of the present invention described above, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance. Consequently, the optical device is advantageous in that the transmission quality can be improved, power can be saved, and the performance of the optical modulator can be improved.
Preferably, the first and second substrates are formed from a common substrate, and the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator are integrated as a unitary member. Further, preferably the substrate is made of LiNbO3, and the substrate made of LiNbO3 is Z-axis cut.
The optical device may be constructed such that one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as an optical device having the characteristics of the optical device according to the first aspect of the present invention while the electrode means of the other of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator includes a signal electrode and a grounding electrode, and the grounding electrodes of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator are formed as a common grounding electrode.
Alternatively, the optical device may be constructed such that one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as an optical device having the characteristics of the optical device according to the first aspect of the present invention while the electrode means of the other of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as a dual electrode having two signal electrodes or as a single electrode having a single signal electrode.
Further, the optical device may be constructed such that the clock generator applies a clock signal of a frequency equal to one half the transmission rate of output light of the optical device per unit time to one of the first and second electrodes to produce an RZ signal of a transmission rate equal to the transmission rate of output light of the optical device per unit time. In this instance, preferably the transmission rate of output light of the optical device per unit time is at the lowest higher than 10 gigabits per second and the clock signal has a frequency equal to or higher than 5 gigahertz.
Alternatively, the optical device may be constructed such that the clock generator applies a clock signal of a frequency equal to the transmission rate of output light of the optical device per unit time to one of the first and second electrodes to produce an RZ signal of a transmission rate equal to the transmission rate of output light of the optical device per unit time. In this instance, preferably the transmission rate of output light of the optical device per unit time is at the lowest higher than 10 gigabits per second and the clock signal has a frequency equal to or higher than 10 gigahertz.
Preferably, the grounding electrode has a cutaway portion formed therein and light loss reduction means for reducing the loss of light propagated is formed on a connection path between the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator.
According to a sixth aspect of the present invention, there is provided an optical device, comprising a Mach-Zehnder first optical modulator including a first substrate having an electro-optical effect, a Mach-Zehnder first optical waveguide formed on the first substrate, and first electrode means formed on the first substrate for controlling light which propagates in the first optical waveguide, a Mach-Zehnder second optical modulator connected in cascade connection to the Mach-Zehnder first optical modulator and including a second substrate having an electro-optical effect, a Mach-Zehnder second optical waveguide formed on the second substrate and connected to the first optical waveguide, and second electrode means formed on the second substrate for controlling light which propagates in the second optical waveguide, a clock generator connected to a first one of the first and second electrode means for applying a clock signal to the first one of the first and second electrode means to produce an RZ signal, and an NRZ data signal generator connected to a second one of the first and second electrode means for supplying an NRZ data signal to the second one of the first and second electrode means, at least one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator being formed as an optical device having the characteristics of the optical device according to the third aspect of the present invention.
With the optical device, since at least one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed so as to have the characteristics of the optical device according to the third aspect of the present invention described above, the amplitude values of the voltages (drive voltages) to be applied to the first and second signal electrodes can be reduced while maintaining a favorable modulation performance. Consequently, the optical device is advantageous in that the transmission quality can be improved and the power can be saved and besides the performance of the optical modulator can be improved.
Preferably, the first and second substrates are formed from a common substrate, and the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator are integrated as a unitary member. Further, preferably the substrate is made of LiNbO3, and the substrate made of LiNbO3 is Z-axis cut.
The optical device may be constructed such that one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as an optical device having the characteristics of the optical device according to the third aspect of the present invention while the electrode means of the other of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator includes a signal electrode and a grounding electrode, and the grounding electrodes of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator are formed as a common grounding electrode.
Further, the optical device may be constructed such that one of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as an optical device having the characteristics of the optical device according to the third aspect of the present invention while the electrode means of the other of the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator is formed as a dual electrode having two signal electrodes or as a single electrode having a single signal electrode.
The optical device may be constructed also such that the clock generator applies a clock signal of a frequency equal to one half the transmission rate of output light of the optical device per unit time to one of the first and second electrodes to produce an RZ signal of a transmission rate equal to the transmission rate of output light of the optical device per unit time. In this instance, preferably the transmission rate of output light of the optical device per unit time is at the lowest higher than 10 gigabits per second and the clock signal has a frequency equal to or higher than 5 gigahertz.
Alternatively, the optical device may be constructed such that the clock generator applies a clock signal of a frequency equal to the transmission rate of output light of the optical device per unit time to one of the first and second electrodes to produce an RZ signal of a transmission rate equal to the transmission rate of output light of the optical device per unit time. In this instance, preferably the transmission rate of output light of the optical device per unit time is at the lowest higher than 10 gigabits per second and the clock signal has a frequency equal to or higher than 10 gigahertz.
Preferably, the grounding electrode has a cutaway portion formed therein and light loss reduction means for reducing the loss of light propagated is formed on a connection path between the Mach-Zehnder first optical modulator and the Mach-Zehnder second optical modulator.
According to a seventh aspect of the present invention, there is provided an optical device, comprising a substrate having an electro-optical effect, a single optical waveguide formed on a surface of the substrate, means for defining a groove provided on the surface of the substrate in the proximity of the optical waveguide, a signal electrode provided on the optical waveguide, and a grounding electrode provided in the groove.
With the optical device, the amplitude value of the voltage to be applied to the signal electrode can be reduced by the grounding electrode. Therefore, the optical device is advantageous in that the power consumption required to drive the signal electrode can be reduced, operation cost of the optical communication system in which the optical device is incorporated can be reduced, and the performance of the device can be improved.
According to an eighth aspect of the present invention, there is provided an optical device, comprising a substrate having an electro-optical effect, a pair of optical waveguides formed on a surface of the substrate, means for defining a plurality of grooves provided on the surface of the substrate in the proximity of the pair of optical waveguides, a signal electrode provided on the pair of optical waveguides, and a grounding electrode provided in the plurality of grooves.
With the optical device, the amplitude value of the voltage to be applied to the signal electrode can be reduced by the grounding electrode. Therefore, the optical device is advantageous in that the power consumption required to drive the signal electrode can be reduced, operation cost of the optical communication system in which the optical device is incorporated can be reduced, and the performance of the device can be improved.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.